Render texture without render texture?

Hey, I'm currently working on [some lighting] stuff that currently renders a screen-sized texture and then blends it with the current main screen. However, one of the low-end machines that I'm using for development and test doesn't support any of the render texture extensions [laptop with crappy intel integrated graphics]. I was curious as to whether or not anyone had any way of mimmicking my special case on mediocre hardware. Basically, I need to render two scenes seperately, and then blend them together [using a multiplicative blend...]. Both scenes are exactly the same size, the size of the GL output viewport, though either could be reduced to powers-of-two if need be. I can render each of the two scenes completely independently. Is there any faster way than: rendering the first scene, copying all of the pixels out [glCopyPixels], turning said pixels into a new texture, clearing the output viewport, rendering the second scene, blending the [first scene] texture over the current scene. I don't think it will be of particular help, but this is a full list of the extensions that I do have access to [on this machine][I really don't have too much to work with]:


    GL_ARB_multitexture
    GL_ARB_point_parameters
    GL_ARB_texture_border_clamp
    GL_ARB_texture_compression
    GL_ARB_texture_cube_map
    GL_ARB_texture_env_add
    GL_ARB_texture_env_combine
    GL_ARB_texture_env_dot3
    GL_ARB_texture_env_crossbar
    GL_ARB_transpose_matrix
    GL_ARB_vertex_program
    GL_ARB_window_pos
    GL_EXT_abgr
    GL_EXT_bgra
    GL_EXT_blend_color
    GL_EXT_blend_func_separate
    GL_EXT_blend_minmax
    GL_EXT_blend_subtract
    GL_EXT_clip_volume_hint
    GL_EXT_compiled_vertex_array
    GL_EXT_cull_vertex
    GL_EXT_fog_coord
    GL_EXT_multi_draw_arrays
    GL_EXT_packed_pixels
    GL_EXT_packed_pixels_12
    GL_EXT_rescale_normal
    GL_EXT_secondary_color
    GL_EXT_separate_specular_color
    GL_EXT_stencil_wrap
    GL_EXT_texture_compression_s3tc
    GL_EXT_texture_env_add
    GL_EXT_texture_env_combine
    GL_EXT_texture_lod_bias
    GL_EXT_texture_filter_anisotropic
    GL_3DFX_texture_compression_FXT1
    GL_IBM_texture_mirrored_repeat
    GL_NV_blend_square
    GL_NV_texgen_reflection
    GL_WIN_swap_hint
    GL_SGIS_generate_mipmap

Although I don't care if the implementation runs in realtime, i'm still quite interested in making this part work as fast as possible on this old hardware [as I could just update one of the images at a much lower rate]. Or, if there are any other non-rendertexture extensions that would make my job easier on other older hardware, then it would be great if you'd be able to mention them. Cheers, CJM

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